I often find that having a strong theme makes the planning process easier. Here is a big list of themes I put together whether for camp, ceremonies, picnics etc.
Apples, Astronomy, Alberta, Aliens, Alphabet, Air, Adventure, Arctic, Atlantis, Aloha, Amazing Race
Bugs, Be Prepared, British Columbia, Beach, Birds, Balloons, Butterflies, Bells, Ball Games, Bridges, Baking
Circus, CSI, Canada, Carnival, Chemistry, Candy, Colours, Cats, Cookies, Challenge, Cowgirls, Careers, Calendar
Dragons, DIY, Dance, Decades, Dogs, Detectives, Diversity, Dinosaurs, Dreams, Desert, Dessert
Earth, Elements, Exploration, Environment, Emotions, Emergencies, Ecology, Endangered Species, Eureka, Entrepreneur
Flowers, Forrest, Fairyland, Friendship, Flight, Farm, Fall, Frogs, Fire, Fruit, Food, Fitness, Folklore, Flower Power, Film, Future, Fish, Fashion
Girl Power, Gold Rush, Gourmet, Goo, Gardens, Gemstones, Gameshows
Holidays, Hollywood, Heritage, History, Health, Happily Ever After, Hogworts, Hiking, Horses
International, Invention, Imagination
Kites, King Arthur
Life Science, Legends, Lizards, Languages, Luha
Mystery, Music, Monsters, Manitoba, Medieval, Magic, Myths, Mental Health, Mountains, Mud, Maps
Neverland, Narnia, New Brunswick, Nova Scotia, Nunavut, North West Territories, Newfoundland, Nursery Rhymes, Nature
Olden Days, Olympics, Owls, Outdoor Cooking, Oz, Oceans, Ontario, Outback, Once Upon a Time
Physics, Pirates, Pajamas, Princesses, Pumpkins, Prairies, Photography, Plants, Peace, PEI, Pioneer, Prints, Pottery, Poetry, Puppets
Quebec, Quests, Quiz, Questions
Robin Hood, Recycling, Rockstar, Rainbow, Rivers, Reptiles
Space, Spa, Superheros, Safari, Saskatchewan, Survivor, Seaside, Spies, Summer, Spring, Secrets, Sports, Songs, Science, Solar System, Snow, Sky
Travel, Time traveler, Tacky, Tall Ships, Talents, Turtles, Toys, Transportation, Team Building
Under the Sea, Unicorns, Underground
Voyagers, Vegetables, Vikings
Western, Weather, Water, Wild, World Centers, Women, Wizard, Winter, Wishes, Wonderland, WAGGGS, Weaving, Wool, Wood
You and Me, Yukon
You can also base a theme around a country such as Australia, Nepal, Japan, Kenya, China, New Zealand, France, Brazil, United Kingdom, India etc.
Or a specific time period 1920s, 1960s etc
Or you can base a theme around books, tv shows or movies but be sure they are ones most if not all your participants are familiar with.
I just wanted to give a shout out the excellent BC Program publication FunFinder. Published roughly 3 times a year in pdf, the back issues are all on their website. Each edition is filled with fun games and activities that can be done with a variety of age groups. The most resent edition (December 2014) is all about using themes at a meeting, special event or camp, to make program more fun and provides two good examples with collections of games around the themes of Wonderland and Mythic Quest.